1. Field of the Invention
The present invention relates to a transmission control apparatus and a transmission control method for an automatic transmission, and more specifically, to a technique for preventing lowering of shift response and shift shock that are attributable to fluctuations of the temperature of an automatic transmission fluid and the like.
2. Description of the Related Art
In general, an automatic transmission for a vehicle is furnished with a planetary gear, which includes power transmission elements such as a sun gear, planetary carrier, etc., and a transmission mechanism, which includes a plurality of hydraulic frictional engagement elements (frictional elements for engagement) such as a wet-type hydraulic multiple plate clutch, hydraulic band brake, etc. In this automatic transmission, a line pressure generated by a hydraulic pump that is driven by means of the crankshaft of an engine is used as a drive source for the frictional engagement elements. The transmission is connected with an electronic control unit, which controls a hydraulic pressure supplied to and discharged from the frictional engagement elements associated with change of gear, thereby controlling the respective operating states of the engagement elements. Thus, an engine torque transmission path for the planetary gear changes, whereupon a desired transmission gear ratio is established.
In the automatic transmission of this electronic-control type, a solenoid-operated hydraulic control valve (hereinafter referred to as solenoid valve) is used for the control of the operating states of the hydraulic frictional engagement elements. More specifically, the hydraulic pressure supply to or discharge from the engagement elements is controlled by regulating the on-off duty ratio of the solenoid valve, whereby the elements are engaged or disengaged. Thus, in making change of gear, a shift shock on the frictional engagement elements being engaged or disengaged is reduced by changing the element to be engaged, that is, by engaging one of those elements associated with the gear change while disengaging the other.
In an upshift from the second-gear speed to the third-gear speed, for example, the frictional engagement element (release-side element) for the second-gear speed is disengaged, while the engagement element (connection-side element) for the third-gear speed is engaged. By thus replacing the engagement element to be engaged, the engine torque transmission path can be smoothly changed to complete the upshift.
A wet-type hydraulic multiple plate clutch is widely used as one such frictional engagement element. This clutch includes drive plates and driven plates arranged alternately, a clutch piston, and a return spring for urging the piston toward a return position. When a hydraulic pressure is supplied to the clutch piston, the piston moves against the urging force of the return spring, thereby pressing the adjacent plates against one another, so that the clutch is engaged. When the hydraulic pressure supply to the piston is stopped, the piston is returned by means of the urging force of the return spring, so that the adjacent plates are separated from one another to disengage the clutch.
In the wet-type hydraulic multiple plate clutch, the drive and driven plates are placed in an automatic transmission fluid (ATF). If the plates are located close to one another, drag torque is generated between the adjacent plates even when the clutch is disengaged or released. To avoid this, the clutch is designed so that a required clearance is formed between each two adjacent plates when it is released. If the moving speed of the clutch piston is too low, therefore, the engagement of the clutch takes much time, so that the shift response is lowered. If the moving speed of the piston is too high, in contrast with this, the clutch is engaged so suddenly that a great shift shock is generated.
Generally, therefore, "(dead stroke) elimination" is effected such that the solenoid valve is fully opened to increase oil supply to the clutch piston, thereby enabling the piston to move at a relatively high speed, in a dead stroke section of the piston before the start of engagement. When this elimination is completed, the oil supply to the piston is feedback-controlled so that the degree of engagement of the piston is enhanced gradually. Thus, the shift response is improved, and the shift shock is reduced.
The hydraulic pump rotates in synchronism with the crankshaft of the engine, so that the line pressure, in general, is lower than a specified value during idle operation or the like in which the engine rotates at low speed. Owing to variation in workmanship between individual automatic transmissions, moreover, hydraulic characteristics are subject to dispersion. Referring to FIG. 18, there are shown the hydraulic characteristics that depend on the variation in workmanship between individual automatic transmissions. In FIG. 18, the full line represents the median of the hydraulic characteristics, while the broken line and dashed line represent values of deflection attributable to the variation between the individuals. As seen from FIG. 18, the hydraulic characteristics of the individual automatic transmissions are subject to dispersion in the region where the engine speed is low.
If the hydraulic characteristics are subject to dispersion corresponding to the variation or difference in workmanship between the individual automatic transmissions, the line pressure of one transmission differs from that of another, in a shift from a low-speed engine state with the throttle fully restored, e.g., a shift from the N-range to the D- or R-range, coast-downshift, power-off upshift, etc. Inevitably, therefore, time for the dead stroke elimination is inappropriate, and the shift response is lowered.
As is generally known, moreover, the viscosity of the ATF increases at a low temperature such as one immediately after cold start, and lowers at a high temperature such as one after high-speed driving. Thus, the leakage of the ATF from individual sliding parts in the transmission varies, thereby causing the line pressure to fluctuate, depending on the ATF temperature. Referring to FIG. 19, there are shown the hydraulic characteristics that depend on the fluid temperature. In FIG. 19, the full line represents the median of the line pressure after warm-up, while the broken line and dashed line represent values of deflection attributable to fluctuations of the fluid temperature. As seen from FIG. 19, the hydraulic characteristics also vary depending on the fluid temperature.
If the line pressure thus changes with the fluid temperature, it becomes so low at high temperature that the dead stroke elimination cannot be achieved satisfactorily in a fixed time. Accordingly, the start of connection of the clutch is delayed, so that the shift response worsens. At low temperature, on the other hand, the line pressure becomes so high that the dead stroke is suddenly eliminated, and the engagement rapidly advances, inevitably causing a shift shock.
In order to solve the above problem, a learning correction method has been proposed such that the engine speed and the ATF temperature are divided into regions in a low engine speed range such as one for idle operation, appropriate elimination times are set individually for these regions and stored in the electronic control unit, and the stored values are subjected to learning correction for the individual regions.
According to this method of division, however, the engine speed and the ATF temperature have a certain width each, so that learning correction values may possibly fluctuate within one region if the elimination times are subjected to learning correction for the individual regions. To be exact, therefore, the appropriate elimination time for the case where both the engine speed and the ATF temperature are relatively low is different from that for the case where both these variables are relatively high, even though in the same region. Accordingly, the learning correction values vary with every execution of transmission control. If the learning correction values thus vary in the same region, the elimination times never converge, so that a shift shock is also caused.
In this case, each divided region may possibly be subdivided. If this is done, however, massive data must be stored in the control unit.